Double-wall vessels are commonly provided for the storage of low-temperature liquefied gases under pressure or at standard or ambient pressure, with suitable means for insulating the space between the two walls or shells of the vessel.
In an early version, such vessels were provided in accordance with Dewar principles and consisted of inner and outer shells or walls defining a space between them which was maintained under a high vacuum to prevent the development of convection currents tending to cause heat transfer from the outer wall to the inner wall or cold loss from the interior to the exterior. The or each wall could be provided with a highly reflective coating to prevent radiation transfer of energy between the interior and the exterior. Such systems had the obvious disadvantage that loss of the vacuum in the space between the two walls caused an immediate breakdown of the insulation space and substantial heat transfer between interior and exterior. Such heat transfer is disadvantageous for the storage of low-temperature liquefied gases at ambient pressure, causing substantially immediate vaporization of the liquefied gas.
It has been proposed to modify such systems for the storage of such liquefied gases as hydrogen and helium at temperatures below about 30.degree.K by providing within the evacuated space between the two walls, a so-called superinsulation consisting of alternating layers of woven or nonwoven fabric web of low thermal conductivity and metal foil layers. The interstices of the fabric web further reduce convective flow of heat while the metal foil constitutes a reflection barrier to radiation transfer. While such systems are effective for the purposes stated, they are relatively expensive and difficult to manufacture and are highly sensitive to handling of the vessel. Furthermore, there is the disadvantage that leakage of the vacuum will occur and render the system ineffective. With gases such as hydrogen and helium even the slightest loss of insulating capacity can result in spontaneous vaporization of the stored liquid.
With the development of improved techniques for the storage, transport and utilization of liquefied gas, relatively low cost systems have been devised for insulating vessel and compartments containing the liquefied gas. One such system makes use of a filling of perlite, a porous insulator in the space between the two walls of a double wall vessel. This system, while effective for such relatively high-boiling liquefied gases as methane, are not suitable in the prior-art technique for use in the storage of hydrogen or helium in a liquid state because of thermal leakage and the effects of the heat capacity of the perlite.